Retatrutide: what the trial data actually shows

How it's designed to work

Retatrutide is designed to activate three separate metabolic receptors at once: the glucose-dependent insulinotropic polypeptide (GIP) receptor, the glucagon-like peptide-1 (GLP-1) receptor, and the glucagon receptor. Tirzepatide, by comparison, acts on only two of these (GIP and GLP-1). The added glucagon-receptor activity is thought to increase energy expenditure on top of the appetite-suppressing effects shared with other GLP-1 medicines, which is the pharmacological rationale for its larger weight-loss effect in trials so far.

Phase 2 trial results

The main published efficacy data come from a randomised, placebo-controlled Phase 2 trial in 338 adults with obesity (without type 2 diabetes), conducted from May 2021 to November 2022 and published in the New England Journal of Medicine in August 2023. At 24 weeks, mean body-weight change was −7.2% with 1mg retatrutide, −12.9% with the pooled 4mg groups, −17.3% with the pooled 8mg groups and −17.5% with 12mg, against −1.6% for placebo. At 48 weeks the effect grew further: −8.7% (1mg), −17.1% (pooled 4mg), −22.8% (pooled 8mg) and −24.2% (12mg), against −2.1% for placebo. Source: Jastreboff AM, et al. "Triple–Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial." N Engl J Med. 2023 (PubMed 37366315).

Safety and side effects

In the same Phase 2 trial, the most common adverse events were gastrointestinal — nausea, diarrhoea, vomiting and constipation — occurring more often with retatrutide than placebo and mostly during dose escalation. Reported nausea rose from roughly 14% at the 1mg dose to about 60% at 12mg, illustrating a clear dose-response pattern; a lower 2mg starting dose (versus 4mg) partly reduced these effects. Discontinuation because of adverse events occurred in 6–16% of participants on retatrutide, versus none on placebo. Serious adverse events occurred in 0–6% of retatrutide groups compared with 4% on placebo. Source: Jastreboff AM, et al., N Engl J Med. 2023 (PubMed 37366315), as above.

Phase 3 programme: trial-by-trial results

Eli Lilly's Phase 3 programme for retatrutide is called TRIUMPH. Between May and July 2026 the company reported positive topline results from three separate 80-week, randomised, placebo-controlled trials under that umbrella, each testing the same three doses (4mg, 9mg and 12mg) against placebo. Sources: Eli Lilly and Company investor news releases, 21 May 2026 and 23 Jul 2026.

TRIUMPH-1 — obesity, no diabetes (2,339 participants)

Announced 21 May 2026. At 80 weeks, mean body-weight change was −19.0% (4mg), −25.9% (9mg) and −28.3% (12mg), against −2.2% for placebo. Waist circumference fell by up to 9.5 inches (24.1cm) at 12mg, versus 1.4 inches for placebo. 45.3% of participants on 12mg lost 30% or more of their body weight (versus 0.5% on placebo), and 65.3% of the 12mg group reached a BMI under 30. A 532-person extension in participants with baseline BMI ≥35 continued dosing to 104 weeks, reaching −30.3% (85.0 lbs / 38.5kg) at the top dose. Gastrointestinal side effects were dose-dependent and common — nausea in 28.6–42.4% of retatrutide arms (versus 14.8% on placebo), diarrhoea in 25.2–34.1%, vomiting in 10.6–25.3% — and discontinuation due to adverse events ranged from 4.1% (4mg) to 11.3% (12mg), against 4.9% on placebo. Source: Eli Lilly and Company, TRIUMPH-1 topline results, 21 May 2026.

TRIUMPH-2 — obesity/overweight with type 2 diabetes (1,152 participants)

Announced 23 Jul 2026. As is typical for incretin therapies in people with diabetes, weight loss was smaller than in TRIUMPH-1: −12.7% (4mg), −19.1% (9mg) and −20.8% (12mg) at 80 weeks, against −4.0% for placebo. HbA1c fell by up to 1.6 percentage points on retatrutide (from a baseline of 7.7%), versus 0.2 points on placebo. Gastrointestinal adverse events again predominated, and discontinuation for adverse events ranged from 3.8% to 11.6%, against 4.9% on placebo. Source: Eli Lilly and Company, TRIUMPH-2 topline results, 23 Jul 2026.

TRIUMPH-3 — severe obesity with established cardiovascular disease (1,949 participants)

Also announced 23 Jul 2026. Only the 9mg and 12mg doses were tested in this population: weight loss reached −21.6% (9mg) and −22.6% (12mg) at 80 weeks, against −3.2% for placebo. At the top dose, retatrutide also reduced triglycerides by 37.0%, non-HDL cholesterol by 16.5%, systolic blood pressure by 9.3 mmHg and high-sensitivity C-reactive protein by 51.2%. Source: Eli Lilly and Company, TRIUMPH-3 topline results, 23 Jul 2026.

Cardiovascular outcomes: an important caveat

TRIUMPH-3 also tracked hard cardiovascular events — heart attack, stroke and cardiovascular death — but the headline weight and risk-factor numbers should not be read as proof that retatrutide reduces heart attacks or strokes. On the broader MACE-5 composite, the hazard ratio versus placebo was 0.82 (95% CI 0.55–1.22); on the narrower MACE-3 composite it was 1.12 (95% CI 0.64–1.96) — numerically more events on retatrutide than placebo. Neither result reached statistical significance, reflecting the small number of events (around 50 in total) over 80 weeks in a trial that was designed and sized to test weight loss, not cardiovascular outcomes. Industry press covering the same data described the effect on cardiovascular risk as "less clear" than the weight-loss result. Sources: Eli Lilly and Company, TRIUMPH-3 topline results, 23 Jul 2026; Fierce Biotech, "Lilly's triple-G drug hits 22.6% weight loss, but impact on reducing cardio risk is less clear," Jul 2026.

Liver disease (MASLD/MASH): an early, unconfirmed signal

Separately from the TRIUMPH obesity programme, a Phase 2a substudy in participants with imaging-confirmed metabolic dysfunction-associated steatotic liver disease (MASLD, formerly NAFLD) found that retatrutide cut liver fat content dramatically — by up to roughly 86% at the highest dose — correlating with changes in body weight and abdominal fat. Researchers attribute part of this effect to the drug's glucagon-receptor activity, which promotes the liver's own fat-burning, a mechanism absent from GLP-1-only or dual GIP/GLP-1 medicines such as semaglutide and tirzepatide. Source: Sanyal AJ, et al., "Triple hormone receptor agonist retatrutide for metabolic dysfunction-associated steatotic liver disease: a randomized phase 2a trial." Nature Medicine, 2024. A dedicated Phase 3 trial in MASLD/MASH is under way, but as of September 2026 it has not yet reported results — this is a promising early finding, not a confirmed benefit, and should not be treated as one.

Other indications still in earlier-stage development

Eli Lilly's published trial-design papers describe additional registrational studies of retatrutide in obstructive sleep apnea and knee osteoarthritis, alongside the obesity, diabetes and cardiovascular trials described above. No efficacy results from these studies have been reported at the time of writing. Source: trial-design publication describing the TRIUMPH registrational programme, PubMed 2026.

Regulatory status: not approved anywhere

As of September 2026, retatrutide has no marketing authorisation from the MHRA, the FDA or the EMA. It cannot lawfully be prescribed or sold as a medicine in the UK, and the only legal way to receive it is as a participant in an approved clinical trial. Eli Lilly originally signalled a possible filing by the end of 2026, but has since said it needs more time to gather manufacturing and quality-control data, and now plans to submit a Biologics License Application to the US FDA in the first quarter of 2027. On that revised timeline, UK availability through the MHRA would realistically follow at the earliest in 2027–2028, with any NHS use later still. Source: BioPharma Dive, "Lilly, with new data, to seek FDA approval of obesity drug retatrutide," Jul 2026.

The unlicensed market: bigger, and more dangerous

Despite having no licence anywhere, "retatrutide" is now sold at scale outside the legal medicines supply chain, and enforcement has escalated on two fronts in 2026. First, Eli Lilly itself has gone to court: on 12 August 2026 the company filed six lawsuits against sellers — including medical spas, wellness clinics, compounding pharmacies and online sellers — accusing them of selling black-market retatrutide to the public while mislabelling it "research use only" to dodge medicines regulation, and it has asked social platforms, marketplaces, payment processors and shippers to help cut off the trade. Source: pharmaphorum, "Lilly sues over 'illegal sales' of obesity drug retatrutide," 12 Aug 2026

Second, the products being sold often are not what they claim to be. In the Australian state of Victoria, seven patients were hospitalised for acute liver injury within weeks of using an unregulated product labelled "retatrutide". The treating hepatologist has said a contaminated counterfeit batch is one possible explanation, but was explicit that this is "purely a hypothesis" — no laboratory testing had yet confirmed what the products actually contained. Australia's Therapeutic Goods Administration separately noted that the localised pattern of cases pointed to "a batch problem," and its chief medical advisor confirmed that "the retatrutide that's being sold is not the retatrutide that's currently in clinical trials." The case illustrates a broader point that applies equally in the UK: a product sold under the name "retatrutide" outside the licensed supply chain has no guarantee of containing retatrutide, or any stated dose, at all. Source: ABC News (Australia), "Doctors fear possible contaminated counterfeit peptide batch after seventh case of acute liver damage," 24 Aug 2026

The MHRA has made the same point domestically. In a Northamptonshire operation on 28 May 2026, its Criminal Enforcement Unit recovered around 12,000 doses of unlicensed weight-loss medicines, which it described as "unlicensed weight loss medicines, including retatrutide and tirzepatide," calling it the largest such seizure by the agency to date. The MHRA's Andy Morling said: "Medicines regulation isn't discretionary — it exists to protect people." Source: MHRA / GOV.UK, 29 May 2026. An earlier, larger raid near Northampton in October 2025 also recovered more than 2,000 unlicensed retatrutide and tirzepatide pens alongside raw chemical ingredients. Source: MHRA / GOV.UK, 24 Oct 2025

Because no licensed retatrutide product exists, none of the GPhC-registered pharmacies listed elsewhere on this site can lawfully supply it, and any UK seller offering "retatrutide" for purchase today is, by definition, operating outside the licensed medicines system — the MHRA warns such products are untested for safety, quality or effectiveness and may not even contain what they claim.